Radar antenna with mirror-wheel scanner for sector and conical scanning

ABSTRACT

A quasi-spherical multi-reflector radar antenna has a subreflector and a main reflector with an aperture formed centrally therein. A mirror wheel is positioned behind the aperture while a fixed microwave energy beam is directed axially or radially against the wheel for subsequent reflection to the far field. As the mirror wheel rotates, the microwave energy reflected from the mirror wheel reflects at varying angles to effect unidirectional sector scanning of a high gain pencil or oval beam in the far field.

United States Patent [191 Meek [451 Feb. 19, 1974 RADAR ANTENNA WITHMIRROR-WHEEL SCANNER FOR SECTOR AND CONICAL SCANNING [75] Inventor:James M. Meek, Silver Spring, Md.

[73] Assignee: The United States of America as represented by theSecretary of the Army, Washington, DC.

[22] Filed: Feb. 26, 1973 [21] Appl. No.: 335,874

[52] US. Cl 343/761, 343/781, 343/837, 343/839 [51] Int. Cl H0lq 3/18,HOlq 3/20, HOlq 15/14 [58] Field of Search... 333/781, 758, 761, 766,837, 333/839, 763

[56] References Cited UNITED STATES PATENTS Cook et al. 343/765 PrimaryExaminerEli Lieberman Assistant Examiner-Marvin Nussbaum Attorney,Agent, or Firm-Edward J. Kelly; Herbert Berl; Saul Elbaum [57] ABSTRACTA quasi-spherical multi-reflector radar antenna has a sub-reflector anda main reflector with an aperture formed centrally therein. A mirrorwheel is positioned behind the aperture while a fixed microwave energybeam is directed axially or radially against the wheel for subsequentreflection to the far field. As the mirror wheel rotates, the microwaveenergy reflected from the mirror wheel reflects at varying angles toeffect unidirectional sector scanning of a high gain pencil or oval beamin the far field 10 Claims, 5 Drawing Figures PATENTE FEB 1 9 m4 SHEET 10? 2 RADAR ANTENNA WITH MIRROR-WHEEL SCANNER FOR SECTOR AND CONICALSCANNING The invention described herein may be manufactured, used, andlicensed by or for the United States Government for governmentalpurposes without the payment to me of any royalty thereon.

FIELD OF THE INVENTION The present invention relates to amulti-reflector antenna which produces a sector scan by utilizing fixedmicrowave energy reflection from a rotating mirror wheel. The wheel hasa number of flat mirrors mounted thereon. As the mirrors rotate, thefixed microwave energy beam reflects at varying angles to produce aunidirectional (pencil or oval beam) sector scan. A figuredquasi-spherical main reflector is employed to collimate the beam in thefar field.

BRIEF DESCRIPTION OF THE PRIOR ART The prior art relating to microwaveantennas includes spherical reflector antennas. In the visible opticalspectrum, telescopes have evolved, attributable to Schmidt and Bouwersinvolving multiple reflectors having one or more spherical reflectors.See ANTENNA ENGINEERING HANDBOOK, 1961 Edition (1st Edition) HenryJasik, pg. -12, McGraw Hill Book Co., New York; and ACHIEVEMENTS INOPTICS, Albert Bouwers (Delft, Holland) Elsevier Publishing Co., NewYork, AmsterdamQIn the case of spherical antennas the majority ofexamples have employed a primary feed located in front of the reflector.This has limited the size and complexity of the feed and associatedscanning mechanisms and only relatively simple scanning operations haveheretofore been achieved. Multiple scan modes such as theconical/unidirectional sector combination effecting rapid switchingbetween modes has therefore not previously been accomplished usingspherical, area-aperture reflectors in one assembly. Employment of abulky feed-scanner in front of the main reflector results in largeblockage and attendant degradation of the far field beam pattern.Performance of the system in terms of angle and range acquisition andtracking suffers correspondingly and usefulness for fire control orother applications diminishes correspondingly. In the'accompanyingdescription, bulky feed scanners are located behind the main reflectorand many of the aforementioned problems are eliminated.

BRIEF DESCRIPTION OF THE PRESENT INVENTION The present invention isdirected to a multi-reflector radar antenna which uses a wheelcomprising flat mirror components to generate a sector scan as a fixedbeam of microwave energy impinges against the rotating wheel.

In another embodiment of the present invention, the wheel can be fixedso that a nutating or conical scan can impinge against it therebygenerating a conical scan in the far field. The use of the wheel scanneris believed to be novel. Also, the capability of rapid switching betweena sector scan mode and a conical scan mode lends further uniqueness tothe present invention.

The resultant structure of the present invention provides an improvementin tracking radar antenna systems related to rapid scanningcapabilities, multiple operating modes, rapid mode switching, andconversion of polarization or scan direction.

BRIEF DESCRIPTION OF THE FIGURES FIG.. 1 is a side elevational view ofone embodiment of the present multi-reflector antenna illustrating thedisposition of a mirror wheel relative to a fixed horn that directsradially propagating microwave energy against the rotating mirror wheel.

FIG. 2 is a partial end view of the mirror wheel scanner taken along aplane passing through section line 2 2 of FIG. 1.

FIG. 3 is a section view of another embodiment of the present inventionwhich illustrates a feed horn that may remain fixed while the mirrorwheel rotates to effect unidirectional sector scanning. With the wheelfixed and the horn nutating, a conical scan in the far field can beachieved.

FIG. 4 is a partial end view of the mirror wheel taken along the planepassing through section line 4 4 of FIG. 3.

FIG. 5 is a partial end view of the mirror wheel to illustrate theprinciple of unidirectional scanners.

DETAILED DESCRIPTION OF THE INVENTION Referring to the drawings and moreparticularly FIG. 1 thereof, reference numerals l0 and 12 indicate thesub-reflector and main reflector, respectively, of the antenna. Asindicated by reference numeral 14, a rectangular aperture is formed inthe central portion of the main reflector 12. The long dimension of therectangle is aligned parallel to the direction of scan; in FIGS. 1 and 3the plane of scan motion of the far-field beam is perpendicular to thepage. In FIGS. 2 and 4 the scan plane is left-right.

Behind the main reflector 12 is a pyramidal feed horn 16, FIG. 1. Thishorn is radially disposed with respect to a rotating drum or wheelhaving flat internal mirrors. The wheel is generally indicated byreference numeral 18 and is seen to have a hexagonal cross section asshown in FIG. 2. However, it is to be emphasized that the number of flatinternal mirrors is indicated as six for purposes of illustration onlyand is not intended to be a limitation of the invention. The individualflat internal mirrors are indicated in FIG. 2 by reference numerals 20,22, 24, 26, 28 and 30.

During transmission, microwave energy is fed from the stationary feedhorn 16 in a generally radially outward direction as indicated by ray32. This ray impinges against the flat mirror 24 and reflection occursas indicated by ray 341A separate flat mirror 36 is angularly disposedwith respect to the feed horn 16. More specifically, an opening isformed in the central portion of mirror 36 to allow the feed horn 16 toprotrude therefrom. With mirror 36 in the position shown,-the reflectedray 34 impinges against this mirror 36 at point 38 for furtherreflection, as indicated by 40, for impingement-at point 42 of thesub-reflector l0. Thereafter, the ray is reflected from the rearwardsurface of sub-reflector 10 as indicated by 44. The ray 44 is reflectedfrom the forward surface of the main reflector 12 at point 46 for finaltransmission to the far field, as boundary ray 48. Considering'anopposite boundary ray, reference numeral 54 illustrates another rayimpinging against the flat internal mirror 24 of the mirror wheel 18.This ray is reflected as ray 56 which intercepts the lower portion 58 ofthe angularly oriented stationary flat mirror 38. Reflection results andray 60 falls incident to point 62 on the rearward surface of thesub-reflector 10. Ray 64 is reflected therefrom for impingement againstthe forward surface of the main reflector 12. After final reflection,ray 66 is then transmitted after refraction to the far field as a lowerray boundary relative to the parallel disposed ray 48 after refraction.Rays 66 and 48 are refracted by lens 92 and may be regarded thereafteras parallel boundary rays corresponding to the far field pencil beam.

Considering the microwave energy feed, the stationary feed horn 16curves around an elbow waveguide portion 68 for communication with ahorizontal waveguide portion 70 that is rigidly connected to a waveguideportion 72. This latter mentioned waveguide portion is finally curved toa waveguide portion 74 that terminates in an RF. input 76 via a coupling78. The microwave energy of course is bi-directional. That is to say,the input 76 can be connected to a transmitterreceiver so that thedisclosed antenna system can operate in either the transmit or receivemode.

The following discussion will relate to the drive means for rotating themirror wheel 18.

A hub 80 is suitably attached to the left end of the wheel 18. The hub80 extends to an intermediate shaft portion that is properly journaledby bearings 82. An outer end portion 84 serves as a mounting shaft for agearing arrangement including driven gear 86 and a driving pinion gear88 that is suitably attached to the output shaft ofa drive motor 90.When the drive motor 90 is energized, the gearing 88, 86 causes rotationof the shaft 84 which is transmitted to hub 80 thereby causing rotationof the mirror wheel 18.

To providesatisfactory focusing for all scan angles, the main reflectormay be shaped or figured to deviate from exactly spherical; hence thename quasi-spherical. In addition a dielectric lens may be installed at92 if desired. t

Linear polarization of the far field radiation may be effected by use ofthe linearly polarized feed 16 and either opaque surfaces at bothreflectors and 12 or transflector at 10, twistflector at 12. Thedirection of sector scan may be chosen, e.g. horizontal or vertical,simply by rotating the antenna system around the bore sight axis to theappropriate orientation. Conversion of the plane of linear polarization,e.g. vertical to horizontal, may be effected by rotating the feed bornabout its long axis to the appropriate orientation. (When thetwistflector-transflector design is employed, the wire grids in thesubreflector and mainreflector must be oriented so as to correspond tothe feed polarization. Details of correct transflector-twistflectordesign may be found in the literature.) The relative ease of selectingor changing polarization independently of scan direction (i.e. byrotating the feed) is a unique advantage of the mirror scanner.

The described embodiment of the present invention is most practical foroperating in the unidirectional sector scan mode. FIGS. 3 and 4illustrate a modification to the described invention which allows theantenna system to be utilized in the wide sector scan mode or theconical scan mode. Otherwise stated, in a single antenna system of theSchwarzschild type, the system can be switched between the sectoralscanning mode and the conical scanning mode.

Considering the embodiment illustred in FIG. 3, flat sub-reflector 90,quasi-spherical main reflector 92, and lens 145, constitute the antennaconfiguration. A rectangular aperture 94 is centrally formed in thereflector 92. Behind the aperture 94 is a mirror wheel generallyindicated by 96 that is a bit different from the previously describedmirror wheel 18 of FIG. 1. As clearly illustrated in FIG. 4, the mirrorwheel 96 is comprised of a set of pyramidally disposed, flat, internalmirrors 98 that form the circumference of the wheel 96. Referencenumeral 98 typifies a single mirror component in the wheel.

A pyramidal or conical feed horn 100, FIG. 3, is mounted in axiallyspaced relationship from the axis of the wheel 96. To achieve sectorscanning, the feed horn 100 remains stationary while the wheel 96rotates. When the wheel rotates, a ray 102 emanating from the hornduring transmission impinges upon mirror 98 at point 104. The ray isreflected as illustrated by 106 to a vertically downward direction. Anangularly oriented fixed mirror 108 intercepts the ray at an upperportion 110. The point of incidence is indicated by reference numeral112. The incident ray is reflected as ray 114. The ray 114 impinges, atpoint 116, on the rearward contoured surface of the subreflector 90.After reflection, the ray 118 impinges at point 120 against the forwardsurface of the main reflector 92. A final reflection occurs as indicatedby ray 122 which characterizes the upper limit of a beam transmitted tothe far field. The second limit of such a beam is initially generated byray 124 that emanates from the horn 100. The ray hits the mirror 98 andis reflected at 126 as ray 128. The mirror 108 intercepts this ray atthe point 130 whereat the ray is reflected (132) for impingement againstthe rearward surface of the sub-reflector 90, at point 134. Ray 136 isreflected from the sub-reflector until it impinges upon the lower point138 of the main reflector 92. A final reflected ray 140 results fortransmission to the far field. Ray 140 passes through lens andrepresents the lower ray limit of the transmitted beam in aunidirectional sector scan, scanning perpendicular to the plane of thepage.

As indicated in FIG. 3, each component or individual mirror of the wheelhas a wedge 142 attached to the rearward surface thereof. This wedgeserves as a means for mounting its respective mirror to a bearingassembly 14.

To further illustrate the principle involved in the operation of theunidirectional scanners, reference is made to FIG. 5. A portion of ascanner wheel 1 is shown rotating schematically about center 2 which iscoincident with the mouth of a stationary feed horn. The microwaveenergy emanating fromsaid horn has a principal ray 2 3 intersecting amirror on wheel 1 at point 3. Wheel 1 is in continuous clockwise motion,direction 4, so that the reflected principal ray will sweep through anare 5 5 centered approximately at point 3. To explore this action inmore detail, consider the motion of the reflected principal ray startingwith one corner of the wheel at arbitrary position 6 and rotating toposition 6. The reflected ray will occupy successively positions 3 7, 35, 3 5, and 3 7. The intervals between these positions represent aportion of a scan, a scan retrace (or flyback), and another portion of ascan, successively. The total scan angle is represented by are 5 5'. Thescan retrace of flyback transition occurs more or less instantaneouslyas the ray 2 3 intercepts corner 6 in passing toward position 6'. Inactuality, the principal ray represents the center of a relativelynarrow feed beam and the transition can be considered complete when thehigh intensity beam has passed from one mirror to the next. Duringtransition, the beam is momentarily split so that part of the energy ispropagated in the direction of ray 3 5' and the residual along 3 5. Thusa beam decay is occurring at 3 5 while a buildup is occurring at 3 5,the time interval being only a small fraction of the scan interval. Theoperating principle may now be applied to the embodiments of FIGS. 1through 4, wherein the scanning beam is further collimated, asdescribed, and directed to the far field as a scanning pencil beam bythe main and sub-reflectors as shown. At the end of each unidirectionalscan, the far field beam will exhibit the intensity decay (at end ofscan) and simultaneous buildup (at begin of scan) as a result of thebeam splitting effect within the scanner.

Conventional means, such as gearing, pulleys, or the like (not shown)cause rotation of the mirror wheel.

The above-described embodiment dealing with FIGS. 3 and 4 were directedto the production of a relatively wide angle unidirectional sector scan.However, the invention can be employed to produce a conical scan orsteady track mode. To accomplish this, rotation of the mirror wheel 96is stopped. Instead, the feed horn 100, is mounted to that its principalray axis is tilted slightly with respect to the asis of a journalsupporting the horn assembly and rotationally driven by a motor (notshown). A microwave rotary joint permits horn rotation. The tilt angleof the feed combined with rotation provides conical scanning in the farfield, the beam collimation being provided in the same manner as in thesector scan mode. If monopulse tracking would be desired instead of aconical scan. then the nutating horn 100 could be replaced by four ormore adjacently placed fixed horns that define the corners of arectangle. Thus, with no mechanical motion occurring in the system,monopulse range and angle detection or sequential lobing may be effectedin a conventional manner.

FIG. 4 illustrates the inclusion of ten flat mirrors in the mirrorwheel. However, as in the embodiment disclosed in connection with FIGS.1 and 2, this number of mirrors is merely exemplary and is not intendedto be a limitation. Actually, for both embodiments, the number ofmirrors is determined by the scan angle desired during unidirectionalsector scanning as well as other desired beam characteristics. Forexample, to obtain a relatively small transition or flyback" time periodcompared to scan period, the feed beam illumination should cover a smallpercentage of a particular mirror in a wheel. This requires either anarrow feed beam width or broad mirrors. The feed beam width affects thefar-field radiation pattern (gain, side lobes, etc.). The use of broadmirrors results in either a larger scanner wheel or fewer mirrors. Thelatter results in increasing'the scan angle in the far field. The scanangle is in turn limited by the necessity of keeping the primary beamfrom excessively spilling over the reflector edges.

The scan angle magnitude in the scanner wheel system may be controlledby the angular positioning of the wheel mirrors relative to the feedaxis. For example,

the angle, FIG. 1, results in a 2/] ratio of angle of reflected energyrotation to wheel rotation, while the 45 angle, FIG. 3, results in a 1/]ratio.

From this discussion, it should be evident that careful selection of theinterrelated design parameters is necessary to achieve optimum ordesired overall antenna performance. In the case of the arrangementshown in FIGS. 1 and 2, feed blockage also must be considered if a largefeed horn is used.

It should be understood that the invention is not limited to the exactdetails of construction shown and described herein for obviousmodifications will occur to persons skilled in the art.

' Wherefore, I claim the following:

1. An antenna system having a flat sub-reflector and a quasi-sphericalmain reflector with a centrally formed aperture therein, the systemcomprising:

a hollowed mirror wheel having its axis concentric with the center ofthe aperture and located behind the aperture;

means mounted adjacent the. wheel for feeding a steady flow of microwaveenergy to the wheel which is reflected therefrom; and

means for further reflecting the energy, reflected from the wheel,through the aperture to the main and sub-reflectors for transmission tothe far field.

2. The subject matter of claim 1 wherein the mirror wheel is comprisedof a cylindrical body having flat plate internal mirror componentsthereon.

3. The structure of claim 2 wherein the means for further reflectingenergy, reflected from the wheel, comprises a stationary mirror mountedinwardly of the mirror wheel at an angular orientation relative to anaxis of the cylindrical body.

4. The system of claim 3 wherein the means feeding a steady flow ofmicrowave energy to the mirror wheel comprising a stationary feed hornwhich protrudes through an opening in the center of the stationarymirror.

5. The subject matter of claim 4 together with means connected to themirror wheel for rotating the wheel and producing a unidirectionalsector scan that is subsequently transmitted to the far field.

6. The system of claim 1 wherein the mirror wheel is comprised of aplurality of trapezoidal-shaped flat mirror components forming afrusto-conical hollowed body.

7. The subject matter recited in claim 6 wherein the means for furtherreflecting the energy, reflected from the wheel, comprises a stationarymirror mounted inwardly of the mirror wheel at an angular orientationrelative to an axis of the frusto-conical hollowed body.

8. The system of claim 7 wherein the means feeding a steady flow ofmicrowave energy to the mirror wheel comprises a stationary hornpositioned adjacent the body so that the energy emitted therefrom flowsin a path parallel to the axis of the frusto-conical body.

9. The subject matter defined in claim 8 together with bearing meansmounted to the mirrored frustoconical body to permit the rotationthereof while the statonary horn feeds a steady flow of energy;

thereby effecting the generation of a unidirectional sector scan in thefar field.

10. The subject matter defined in claim 8 together with means mounted tothe stationary horn for imparting nutating motion to the horn while themirrored frusto-conical body remains stationary thereby producing aconical scan in the far field.

1. An antenna system having a flat sub-reflector and a quasispherical main reflector with a centrally formed aperture therein, the system comprising: a hollowed mirror wheel having its axis concentric with the center of the aperture and located behind the aperture; means mounted adjacent the wheel for feeding a steady flow of microwave energy to the wheel which is reflected therefrom; and means for further reflecting the energy, reflected from the wheel, through the aperture to the main and sub-reflectors for transmission to the far field.
 2. The subject matter of claim 1 wherein the mirror wheel is comprised of a cylindrical body having flat plate internal mirror components thereon.
 3. The structure of claim 2 wherein the means for further reflecting energy, reflected from the wheel, comprises a stationary mirror mounted inwardly of the mirror wheel at an angular orientation relative to an axis of the cylindrical body.
 4. The system of claim 3 wherein the means feeding a steady flow of microwave energy to the mirror wheel comprising a stationary feed horn which protrudes through an opening in the center of the stationary mirror.
 5. The subject matter of claim 4 together with means connected to the mirror wheel for rotating the wheel and producing a unidirectional sector scan that is subsequently transmitted to the far field.
 6. The system of claim 1 wherein the mirror wheel is comprised of a plurality of trapezoidal-shaped flat mirror components forming a frusto-conical hollowed body.
 7. The subject matter recited in claim 6 wherein the means for further reflecting the energy, reflected from the wheel, comprises a stationary mirror mounted inwardly of the mirror wheel at an angular orientation relative to an axis of the frusto-conical hollowed body.
 8. The system of claim 7 wherein the means feeding a steady flow of microwave energy to the mirror wheel comprises a stationary horn positioned adjacent the body so that the energy emitted therefrom flows in a path parallel to the axis of the frusto-conical body.
 9. The subject matter defined in claim 8 together with bearing means mounted to the mirrored frusto-conical body to permit the rotation thereof while the stationary horn feeds a steady flow of energy; thereby effecting the generation of a unidirectional sector scan in the far field.
 10. The subject matter defined in claim 8 together with means mounted to the stationary horn for imparting nutating motion to the horn while the mirrored frusto-conical body remains stationary thereby producing a conical scan in the far field. 